Method and apparatus to compensate for polarization mode dispersion

ABSTRACT

A polarization scrambler and a polarization mode dispersion (PMD) compensation system compensate for PMD on an active optic fiber. The polarization scrambler scrambles a state of polarization of an optical signal that carries user information. The PMD compensation system then receives the optical signal over the active optic fiber. The PMD compensation system measuring a differential group delay and principal states of polarization of the PMD in the active optic fiber. The PMD compensation system then determines a modification of the optical signal based on the differential group delay and the principal states of polarization of the PMD. The PMD compensation system modifies the optical signal in the active optic fiber to compensate for PMD based on the determination of the modification. The PMD compensation system then transmits the optical signal. By measuring the differential group delay and the principal states of polarization, the PMD compensation system adapts to changes in the PMD in the active optic fiber.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/500,092, filed Feb. 8, 2000, entitled “Method and Apparatus toCompensate for Polarization Mode Dispersion,” now U.S. Pat No. 6,459,830B1, which is hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

MICROFICHE APPENDIX

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to the field of communication systems, and inparticular, to a system that compensates for polarization modedispersion in an optic fiber.

2. Description of the Prior Art

In fiber optic communication systems, a fiber that carries opticalsignals contains asymmetries. These asymmetries result in the opticalproperties of the fiber not being the same in all directions. Thus, thefiber is birefringent, where the material displays two different indicesof refraction. This fiber birefringence causes polarization modedispersion (PMD).

PMD is measured like a vector quantity, where a differential group delayis the magnitude of the vector and the principal state of polarization(PSP) are the direction. There are two PSPs associated with PMD. The twoPSPs propagate at slightly different velocities with the distribution ofsignal power varying with time. PMD is a time varying stochastic effect.PMD varies in time with ambient temperature, fiber movement, andmechanical stress on the fibers. Compensating for PMD can be difficultbecause of the time varying nature and randomness of PMD.

Prior systems that involve taking the fiber out of operation tocompensate for PMD are expensive. There have been few systems that haveattempted to compensate for PMD on active fibers. A fiber is active whenthe fiber is operational to exchange user information. One prior systemuses a polarization controller at the transmitter. The polarizationcontroller aligns the input state of polarization of the input opticalsignal to the PSP of the fiber to reduce the signal distortion. Onedisadvantage of this system is the requirement of timely knowledge ofthe PSPs, which is difficult at best. Another disadvantage is the PSP ofthe fibers are different for each receiver. When optical add/drops areinvolved, this system is ineffective.

Another system uses a polarization controller prior to the receiver. Thepolarization controller aligns the polarization of one of the PSPs witha polarization filter. The polarization controller also receives controlsignals from a feedback arrangement. This system processes one of thePSPs which is essentially free from the PMD effects.

Another system uses a polarization controller and a length ofpolarization-maintaining fiber prior to the receiver. The length of thepolarization-maintaining fiber is selected so a fixed value ofdifferential group delay is equal to the average differential groupdelay of the long fiber to minimize the PMD effects. A disadvantage isthis system only works for a fixed value of differential group delay.When differential group delay varies, the system does not fullycompensate for the PMD effects.

Another system monitors the effect of PMD on an input optical signal.The power level of a non-return-to-zero (NRZ) optical signal's spectralcomponent corresponding to one-half of the data rate indicates the PMDin a fiber link. In one example, to monitor the PMD on a 10 Gb/s NRZoptical signal, the system monitors the power of the spectral componentat 5 GHz. This system comprises a narrowband filter centered at 5 GHzfollowed by a square-law detector and a lowpass filter.

One problem is that none of the prior systems track changes in thedifferential group delay, which is a component of PMD. Another problemis the degraded ability to monitor for DGD and PSPs when the input stateof polarization of the input signal is nearly aligned with one of thePSPs. A system is needed that can compensate for PMD which accounts forchanges in the PMD and the problems when the input state of polarizationof the input signal is nearly aligned with one of the PSPs.

SUMMARY OF THE INVENTION

The invention solves the above problems by compensating for PMD. Apolarization scrambler scrambles a state of polarization of an opticalsignal that carries user information. A PMD compensation system thenreceives the optical signal over an active optic fiber. The PMDcompensation system then measures a differential group delay andprincipal states of polarization of the polarization mode dispersion inthe active optic fiber. The PMD compensation system then determines amodification of the optical signal based on the differential group delayand the principal states of polarization of the polarization modedispersion. The PMD compensation system modifies the optical signal inthe active optic fiber to compensate for PMD based on the determinationof the modification. The PMD compensation system then transmits theoptical signal.

In various embodiments of the invention, the PMD compensation systemmeasures the differential group delay and the principal states ofpolarization of the PMD in the active optic fiber by estimating thedifferential group delay and the principal states of polarization of thePMD in the active optic fiber. The PMD compensation system modifies theoptical signal by changing the polarization state of the optical signal.The PMD compensation system modifies the optical signal by changing thedifferential group delay of the PMD in the active optic fiber.

Advantageously, the invention adapts to the time varying nature of thePMD in the active optic fiber by measuring the differential group delayand the principal states of polarization. Also, the invention is appliedto active optic fibers so the fiber optic communication system does nothave to be taken out of operation to compensate for PMD. The inventionadvantageously scrambles a state of polarization of the optical signalto greatly improve the measurement of the differential group delay andthe principal states of polarization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system level block diagram of an example of the invention.

FIG. 2 is a flow chart of the operation of a polarization scrambler anda PMD compensation system in an example of the invention.

FIG. 3 is a system level diagram of a fiber optic communication systemwith a PMD compensation system including a feedback arrangement in anexample of the invention.

FIG. 4 is a flow chart of an operation of a compensation algorithmsystem in an example of the invention.

A particular reference number refers to the same element in all of theother figures.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a system level block diagram of a fiber opticcommunication system 100 in an example of the invention. In FIG. 1, atransmitter 102 is connected to a polarization scrambler 104. Thepolarization scrambler 104 is connected to a PMD compensation system 108via an active optic fiber 106. The PMD compensation system 108 isconnected to a receiver 110.

The transmitter 102 could be any device configured to transmit opticalsignals. The transmitter 102 typically modulates the optical signals tocarry user information. The receiver 110 could be any device configuredto receive optical signals. The receiver 102 typically derives data fromthe optical signals.

The polarization scrambler 104 is any device or group of devicesconfigured to scramble the state of polarization of the optical signalthat carries user information. The PMD compensation system 108 is anydevice or group of devices configured to (1) receive the optical signalover the active optic fiber 106, (2) measure a differential group delayand principal states of polarization of the polarization mode dispersionin the active optic fiber 106, (3) determine a modification of theoptical signal based on the differential group delay and the principalstates of polarization of the polarization mode dispersion, (4) modifythe optical signal in the active optic fiber 106 to compensate forpolarization mode dispersion based on the determination of themodification, and (5) transmit the optical signal.

In operation, the transmitter 102 transmits the optical signal to thepolarization scrambler 104. FIG. 2 shows a flow chart of the operationof the the polarization scrambler 104 and the PMD compensation system108 in an example of the invention. FIG. 2 begins with step 200. In step202, the polarization scrambler 104 scrambles a state of polarization ofthe optical signal. The PMD compensation system 108 then receives theoptical signal over the active optic fiber 106 in step 204. In step 206,the PMD compensation system 108 measures a differential group delay andprincipal states of polarization of the PMD in the active optic fiber106. In step 208, the PMD compensation system 108 then determines amodification of the optical signal based on the differential group delayand the principal states of polarization of the PMD. In step 210, thePMD compensation system 108 modifies the optical signal in the activeoptic fiber 106 based on the determination made in step 208 tocompensate for PMD. The PMD compensation system 108 then transmits theoptical signal. The operation of the PMD compensation system 108 ends instep 212. The receiver 110 receives the optical signal from the PMDcompensation system 108.

FIG. 3 discloses one embodiment of the invention, but the invention isnot restricted to the configuration provided below. Those skilled in theart will appreciate numerous variations in a fiber optic communicationsystem configuration and operation that are within the scope of theinvention. Those skilled in the art will also appreciate how theprinciples illustrated in this example can be used in other examples ofthe invention.

FIG. 3 depicts a system level diagram of a fiber optic communicationsystem 300 with a PMD compensation system 360 including a feedbackarrangement in an example of the invention. In FIG. 3, the PMDcompensation system 360 comprises a splitter 308, a first polarizationcontroller 310, a PMD emulator 312, a photodetector 320, a RF signalprocessor 322, a compensation algorithm system 330, a link 332, a link334, a link 336, a link 338, a second polarization controller 340, and aPMD emulator 342. A transmitter 302 is connected to a polarizationscrambler 304. The polarization scrambler 304 is connected to thesplitter 308 via an active optic fiber 306. The splitter 308 isconnected to the first polarization controller 310 and the secondpolarization controller 340.

The first polarization controller 310 is connected to the PMD emulator312. The PMD emulator 312 comprises a splitter 314, a link 316, and adelay component 318. The first polarization controller 310 is connectedto the splitter 314. The splitter 314 is coupled to the photodetector320 via the link 316 and is coupled to the photodetector 320 via thedelay component 318. The photodetector 320 is connected to the RF signalprocessor 322. The RF signal processor 322 comprises a lowpass filter324, a square law detector 326, and a bandpass filter 328. Thephotodetector 320 is connected to the bandpass filter 328. The bandpassfilter 328 is connected to the square law detector 326. The square lawdetector 326 is connected to the lowpass filter 324. The low pass filter324 is connected to the compensation algorithm system 330. Thecompensation algorithm system 330 is coupled to the first polarizationcontroller 310 via the links 332 and 334. The compensation algorithmsystem 330 is connected to the PMD emulator 312 and the PMD emulator342. The compensation algorithm system 330 is coupled to the secondpolarization controller 310 via the links 336 and 338.

The second polarization controller 340 is connected to the PMD emulator342. The PMD emulator 342 comprises a splitter 344, a link 346, and adelay component 348. The second polarization controller 340 is connectedto the splitter 344. The splitter 344 is coupled to the receiver 350 viathe link 346 and is coupled to the receiver 350 via the delay component348.

A process path comprises the first polarization controller 310, the PMDemulator 312, the photodetector 320, the RF signal processor 322, andthe compensation algorithm system 330. The components in the processpath collectively measure the differential group delay and determine themodification of the optical signal based on the differential groupdelay. The data path comprises the second polarization controller 340and the PMD emulator 342. The components in the data path collectivelymodify the optical signal based on the determination from the processpath.

In operation, the transmitter 302 transmits the optical signal to thepolarization scrambler 304. In some embodiments of the invention, thetransmitter 302 includes a laser diode. The polarization scrambler 304then scrambles the state of polarization of the optical signal thatcarries user information. Scrambling the state of polarization of theoptical signal provides the greatest probability of having the powersplit between the two PSPs, while all of the power propagating along oneof the PSP has the lowest probability. When the power is equally splitbetween the two PSPs, the measurements of the DGD and PSPs are greatlyimproved. Thus, the polarization scrambler's 304 scrambling of theoptical signal greatly improves the measurements of the DGD and PSPs. Inone embodiment of the invention, the polarization scrambler's 304 rateof scrambling is greater than the response time of the low pass filter324 to provide each sample of the low pass filter 324 multiple aligmentsof the optical signal.

The splitter 308 then receives the optical signal over the active opticfiber 306. In some embodiments of the invention, the active optic fiber306 includes chromatic dispersion compensation systems, opticalamplifiers, or multiple spans of optical fiber. Also, in otherembodiments, the active optic fiber 306 carries wavelength divisionmultiplexed (WDM) optical signals. The WDM optical signals arede-multiplexed prior to entering the splitter 308 in order for theoperation of the PMD compensation system 360 to work properly. The PMDcompensation system 360 may be required for each channel for a WDMsignal.

The splitter 308 splits the optical signal. The splitter 308 transfersthe optical signals to the first polarization controller 310 and thesecond polarization controller 340. The first polarization controller310 receives the optical signal from the splitter 308. The firstpolarization controller 310 then changes the state of polarization ofthe optical signal based on signals received from the links 332 and 334.In one embodiment, the first polarization controller 310 aligns theactive fiber link's 306 output principal state of polarization with theprincipal state of polarization of the PMD emulator 312. The firstpolarization controller 310 transfers the optical signal to the PMDemulator 312. The splitter 314 in the PMD emulator 312 receives theoptical signal and splits the optical signal into two optical signalswith orthogonal polarizations. The splitter 314 transmits one opticalsignal with the orthogonal polarization to the link 316. The splitter314 also transmits the other optical signal with the orthogonalpolarization to the delay component 318. The delay component 318 delaysthe optical signal with the orthogonal polarization based on signalsreceived from the compensation algorithm system 330. The PMD emulator312 recombines the two optical signals with orthogonal polarizationsfrom the link 316 and the delay component 318 before transferring theoptical signal to the photodetector 320.

The photodetector 320 receives the optical signal. The photodetector 320converts the optical signal to an electrical signal before transferringthe electrical signal to the RF signal processor 322. The bandpassfilter 328 receives the electrical signal. The bandpass filter 328 is anarrow pass band centered at half the signal data rate. The bandpassfilter 328 then transfers the electrical signal to the square-lawdetector 326. The square-law detector 326 processes the electricalsignal and transfers the electrical signal to the lowpass filter 324.The lowpass filter 324 receives the electrical signal. The lowpassfilter 324 converts the electrical signal to a control signal beforetransferring the control signal to the compensation algorithm system330.

FIG. 4 is a flow chart of an operation of the compensation algorithmsystem 330 in an example of the invention. FIG. 4 begins in step 400. Instep 402, the compensation algorithm system 330 sets the emulateddifferential group delay of the PMD emulator 312 to an arbitrary butfixed value. In this embodiment, the initial emulated differential groupdelay is 15 picoseconds. Also, the compensation algorithm system 330sets the initial PMD emulator 342 differential group delay to 0picoseconds. In step 404, the compensation algorithm system 330 readsthe power of the control signal. In step 406, the compensation algorithmsystem 330 checks if the power at the control signal is at a minimum.

If the power at the control signal is not at a minimum, the compensationalgorithm system 330 proceeds to step 408. In step 408, the compensationalgorithm system 330 changes the first polarization controller 310values via the link 332 and the link 334. The link 332 carries signalsthat control the θ value of the first polarization controller 310. Thelink 334 carries signals that control the φ value of the firstpolarization controller 310. Once the first polarization controller 310values are changed, the compensation algorithm system 330 returns tostep 404.

If the power at the control signal is at a minimum, the compensationalgorithm system 330 proceeds to step 410. In step 410, the compensationalgorithm system 330 varies the emulated differential group delay in thePMD emulator 312 and measures the power at the control signal. In step312, the compensation algorithm system 330 determines the maximum powerof the control signal based on the measurements from step 410. Thecompensation algorithm system 330 estimates the differential group delayof the active optic fiber 306 by using the differential group delayvalue at the maximum power of the control signal. The compensationalgorithm system 330 then sets the emulated differential group delayvalue of the PMD emulator 312 with the estimated active optic fiber 306differential group delay value.

In step 414, the compensation algorithm system 330 reads the power ofthe control signal. In step 416, the compensation algorithm system 330checks if the power at the control signal is at a maximum. If the powerat the control signal is not at a maximum, the compensation algorithmsystem 330 proceeds to step 418. In step 418, the compensation algorithmsystem 330 changes the first polarization controller 310 values via thelink 332 and the link 334. Once the first polarization controller 310values are changed, the compensation algorithm system 330 returns tostep 414.

If the power at the control signal is at a maximum, the compensationalgorithm system 330 proceeds to step 420. In step 420, the compensationalgorithm system 330 changes the polarization controller values and thedifferential group delay value from the data path from the polarizationcontroller values and the emulated differential group delay value fromthe process path. The link 336 carries signals that control the θ valueof the second polarization controller 340. The link 338 carries signalsthat control the φ value of the second polarization controller 340. Thecompensation algorithm system 330 sets the θ value of the link 336 tothe θ value of the link 332. The compensation algorithm system 330 setsthe φ value of the link 338 to the φ value of the link 334. Thecompensation algorithm system 330 sets the differential group delay ofthe PMD emulator 342 to the emulated differential group delay of the PMDemulator 312. The operation of the compensation algorithm ends at step422 and returns to step 400 to continually compensate for PMD.

The second polarization controller 340 receives the optical signal fromthe splitter 308. The second polarization controller 340 then changesthe state of polarization of the optical signal based on signalsreceived from the link 336 and the link 338. In one embodiment, thesecond polarization controller 340 aligns the active fiber link's 306principal state of polarization with the principal state of polarizationof the PMD emulator 342. The second polarization controller 340transfers the optical signal to the PMD emulator 342. The splitter 344in the PMD emulator 342 receives the optical signal and splits theoptical signal into two optical signals with orthogonal polarizations.The splitter 344 transmits one optical signal with the orthogonalpolarization to the link 346. The splitter 344 also transmits the otheroptical signal with the orthogonal polarization to the delay component348. The delay component 348 delays the optical signal with theorthogonal polarization based on signals received from the compensationalgorithm system 330. The PMD emulator 342 recombines the two opticalsignals with orthogonal polarizations from the link 346 and the delaycomponent 348 into the optical signal to compensate for PMD. The PMDemulator 342 then transfers the optical signal to the receiver 350. Thereceiver 350 receives the optical signal to derive data from the opticalsignal.

Those skilled in the art will appreciate variations of theabove-described embodiments that fall within the scope of the invention.As a result, the invention is not limited to the specific examples andillustrations discussed above, but only by the following claims andtheir equivalents.

We claim:
 1. A method for measuring polarization mode dispersion in anoptic fiber, the method comprising: varying a state of polarization ofan optical signal to scramble the state of polarization; transmittingthe optical signal over an active optical fiber; receiving the opticalsignal from the active optical fiber; splitting the optical signal intoa processing optical signal and an output optical signal determiningactual principal states of polarization and a differential group delayof the processing optical signal; generating and transmitting aninstruction that indicates the principal states of polarization and thedifferential group delay of the processing optical signal; andcompensating the output optical signal to remove the polarization modedispersion using the instruction.
 2. The method of claim 1 wherein theoptical signal is a wavelength division multiplexed optical signal andthe method further comprising: de-multiplexing the wavelength divisionmultiplexed optical signal; and compensating for the polarization modedispersion of each channel of the wavelength division multiplexedoptical signal.
 3. The method of claim 1 wherein determining theprincipal states of polarization and the differential group delaycomprises: splitting the processing optical signal into a firstorthogonally-polarized optical signal and a secondorthogonally-polarized optical signal; delaying the secondorthogonally-polarized optical signal; and recombining the first and thesecond orthogonally-polarized optical signals in the processing opticalsignal.
 4. The method of claim 1 wherein determining the principalstates of polarization and the differential group delay comprisesconverting the processing optical signal into an electrical signal. 5.The method of claim 4 wherein determining the principal states ofpolarization and the differential group delay further comprises:generating a control signal by processing the electrical signal with abandpass filter, a square-law detector, and a low pass filter; andrecording a measurement of power of the control signal.
 6. The method ofclaim 5 wherein determining the principal states of polarization and thedifferential group delay further comprises transmitting polarizationparameters to a polarization controller to modify the state ofpolarization of the processing optical signal based on the measurement.7. The method of claim 5 wherein varying the state of polarization ofthe optical signal comprises modulating the state of polarization at afrequency higher than a response time of the low pass filter.
 8. Themethod of claim 1 further comprising modifying the optical signal usingthe instruction to compensate for the polarization mode dispersion. 9.The method of claim 1 wherein varying the state of polarization of theoptical signal comprises modulating the state of polarization in onedimension.
 10. The method of claim 1 wherein varying the state ofpolarization of the optical signal comprises dividing an optical signalpower equally between the principal states of polarization.
 11. Themethod of claim 1 wherein determining the differential group delay ofthe optical signal comprises estimating the differential group delay.12. A system for measuring polarization mode dispersion in an opticfiber, the system comprising: a polarization scrambler configured toreceive an optical signal, modulate a state of polarization of theoptical signal, and transmit the optical signal over an active opticalfiber; and a processing system configured to receive the optical signalfrom the active optical fiber, split the optical signal into aprocessing optical signal and an output optical signal, determine actualprincipal states of polarization and a differential group delay of theprocessing optical signal, generate and transmit an instruction thatindicates the principal states of polarization and the differentialgroup delay of the processing optical signal, and compensating theoutput optical signal to remove the polarization mode dispersion usingthe instruction.
 13. The system of claim 12 wherein the optical signalis a wavelength division multiplexed optical signal and wherein theprocessing system is further configured to de-multiplex the wavelengthdivision multiplexed optical signal and compensate for the polarizationmode dispersion of each channel of the wavelength division multiplexedoptical signal.
 14. The system of claim 12 wherein the processing systemfurther comprises a polarization mode dispersion emulator configured tosplit the processing optical signal into a first orthogonally-polarizedoptical signal and a second orthogonally-polarized optical signal, delaythe second orthogonally-polarized optical signal, and recombine thefirst and the second orthogonally-polarized optical signals.
 15. Thesystem of claim 12 wherein the processing system further comprises apolarization controller configured to modify the state of polarizationof the processing optical signal based on polarization parameters from acompensation algorithm system.
 16. The system of claim 12 wherein theprocessing system further comprises: a photodetector configured toconvert the processing optical signal into an electrical signal; and aradio frequency signal processor comprising a bandpass filter, asquare-law detector, and a low pass filter wherein the radio frequencysignal processor is configured to convert the electrical signal into acontrol signal.
 17. The system of claim 16 wherein the processing systemfurther comprises a compensation algorithm system configured to make ameasurement of a power of the control signal, modify the differentialgroup delay and polarization parameters transmitted to a polarizationcontroller based on the measurement, and generate and transmit theinstruction that indicates the principal states of polarization and thedifferential group delay.
 18. The system of claim 16 wherein thepolarization scrambler is further configured to modulate the state ofpolarization at a frequency higher than a response time of the low passfilter.
 19. The system of claim 12 wherein the polarization scrambler isfurther configured to modulate the state of polarization of the opticalsignal in one dimension.
 20. The system of claim 12 wherein thepolarization scrambler is further configured to split an optical signalpower equally between the principal states of polarization.
 21. Thesystem of claim 12 wherein the processing system is further configuredto estimate the differential group delay of the optical signal todetermine the differential group delay.